The One After the Superbowl, part 1 - S2-E12
Audio problem: When Phoebe is showing the children her guitar in the library, her voice comes on saying, "I know," when the children are amazed by her guitar. Look carefully - her lips are not moving when she says this.
Answer: The statements that she's contradicting (that it is common, it happens to every guy, and it's not a big deal), are the things that a woman commonly says to a man who is suffering from erectile difficulty, typically to assuage his bruised ego. However, most men do not believe that these statements are true, as evidenced by Chandler's outburst. He's so caught up in the proof that women are lying about it that he gives himself away.
Rooster of Doom